If ADX is below 20, the market is weak or range-bound, so trend-following strategies fail and you should switch to range trading or wait for a break above 20.
Below 20, the market is range-bound or trending so weakly it barely qualifies as a trend, with price probably bouncing between support and resistance without making real progress.
Avoid trend tactics here because breakouts often fail and momentum strategies get chopped up.
Fade extremes inside the range or stand aside until ADX crosses above 20, signifying that a new trend is emerging, then resume directional trades confirmed by the +DI and -DI lines.